Permanent magnet motors have a trade off between having high torque at low speed and having a high speed range. Specifically, if the motor is built such that large amounts of low end torque are produced, the top speed of the motor is reduced. If a high top speed is required, this must come at the expense of low end torque.
For applications such as use in an electrically-powered automobile, it is desirable to have both high torque at low speed and a high top speed of the motor to avoid the necessity for a transmission assembly, which increases cost and complexity, to increase the speed range of the motor and still provide enough torque to accelerate the automobile at an acceptable rate.
Several methods exist presently to address this limitation. However, all are based on reducing the flux density in the stator core. One method of reducing the flux density in the stator core is to vary the air gap between the rotor and stator to alter the flux density in the stator. This method requires complex mechanical assemblies that allow the rotor and stator to change position with respect to one another. For example, in an axial flux electric motor, a mechanical assembly would be required to physically move the rotor and stator axially with respect to one another.
Another method of reducing flux density in the stator core is to introduce currents into the stator that create magnetic fields to oppose the magnetic fields of the permanent magnets on the stator. This has the obvious limitation of reducing the efficiency of the motor. In many applications, such as an electrically-powered automobile, this may not be desirable.